In recent years, some high performance small vehicles, in particular bicycles, have been provided with hydraulic brake systems to increase braking performance. In the case of bicycles, the hydraulic brake systems are typically actuated by a hydraulic operating apparatus such as a brake lever that is attached to the handlebar. The hydraulic operating apparatus typically includes a master piston that is slidably disposed in a cylinder bore of a master cylinder, and which is actuated by a lever. The master cylinder is in fluid communication with a reservoir containing hydraulic fluid.